PROJECT ABSTRACT Mycobacterium tuberculosis (Mtb) and human immunodeficiency virus (HIV) together represent the world's biggest killers. There are an estimated nine million new cases of tuberculosis (TB) diagnosed each year, resulting in 1.4 million deaths annually. It is therefore of paramount importance that we develop novel strategies for controlling Mtb infection. The only currently-licensed vaccine for TB, M.bovis Bacille Calmette Guerin (BCG), is effective against childhood forms of TB, but has limited efficacy against pulmonary TB after adolescence. Thus, development of novel vaccines and therapeutics for TB that will provide improved protection upon Mtb exposure are urgently required. Modern candidate TB vaccines under development have focussed on induction of strong T cell responses, primarily CD4+ T cells producing the cytokine interferon gamma (IFN-?, Th1 cells). More recently, a key role for mucosal interleukin 17A (IL-17) in vaccine-induced protection against TB disease has also been shown. Thus, induction of lung-resident IL- 17-producing CD4+ T cell populations (Th17 cells) by mucosal TB vaccines is also being explored. However, most TB vaccines do not confer sterilizing immunity, instead, inducing a reduction of only ~0.5 to 1.5 logs in lung Mtb burden in animal challenge models. Our new data presented here, show that the lack of sterilizing vaccine-induced immunity to TB vaccines is not due to poor function of vaccine-induced CD4+ T cells, but due to delayed activation and accumulation of recall CD4+ T cell responses in the lung following Mtb infection. Using novel strategies, we show that this bottleneck can be overcome by delivery of activated Mtb antigen (Ag)-pulsed dendritic cells (DCs) into the lungs of vaccinated Mtb-infected mice. DC transfer substantially accelerates the timing of CD4+ T cell accumulation in the lungs, and leads to superior vaccine- induced immunity in Mtb-infected vaccinated mice. Using RNASeq analysis, we have further generated a gene signature in vaccinated mice receiving DC transfer, that is associated with the superior vaccine immunity induced upon Mtb infection. This gene signature reflects upregulation of pathways associated with rapid and effective activation of lung DCs and T cell pathways. Specifically, we found genes associated with activation of CD103+ DC and CD40 pathways upregulated in DC transfer-recipient vaccinated Mtb-infected mice, exhibiting superior Mtb control. In this exploratory proposal, using mouse models of vaccination and Mtb infection, we will investigate whether targeting host lung CD103+ DCs and the CD40 pathway with novel host-directed therapeutics (HDTs) can induce sterilizing vaccine- induced immunity following Mtb infection. These studies will functionally determine the mechanisms that mediate rapid activation of vaccine-induced T cell recall responses to TB. Importantly, these studies will also provide novel HDTs to enhance vaccine responses to either control Mtb burden in infected hosts, or delay TB reactivation in latently-infected individuals.